Attention has been drawn to a hydrogen oxygen fuel cell as a power generation system which gives no substantial adverse effect to the global environment, since the reaction product is only water in principle. With a polymer electrolyte fuel cell which is being studied recently, a very high output is expected at a low operation temperature of from room temperature to about 150° C. In such a case, it is assumed to use, as a fuel, hydrogen gas containing e.g. carbon dioxide, obtained by reforming e.g. methane, methanol or gasoline.
On the other hand, a polymer electrolyte fuel cell has a low operation temperature. Accordingly, exhaust heat can hardly be utilized, for example, as an auxiliary power, and it is utilized only for hot water at best. To offset such a drawback, it is necessary for the polymer electrolyte fuel cell to secure a high output density. Further, for practical application, it is required to secure performance of a high energy efficiency and a high output density even under an operation condition where the fuel and air utilization ratios are high.
As the electrolyte for the polymer electrolyte fuel cell, a perfluorocarbon sulfonic acid type cation exchange membrane, which is an ultrastrong acid, is mainly used, in view of the chemical stability and electric conductivity. When such an acid electrolyte is used, the following reaction occurs at an air electrode, whereby water will be formed.1/2O2+2H++2e−→H2O
Therefore, when the polymer electrolyte fuel cell is operated under such conditions as a low operation temperature, a high current density and a high gas utilization ratio, clogging (flooding) of the electrode is likely to take place due to condensation of steam, at the air electrode where water is formed. Further, gases to be supplied to the fuel electrode and the air electrode, are usually wetted not to dry the ion exchange membrane in order to maintain the electric conductivity of the ion exchange membrane which is the polymer electrolyte. Accordingly, also by such wetted gases, flooding of the electrode is likely to take place.
Accordingly, in order to obtain a stable performance of the fuel cell for a long period of time, it is necessary to impart water repellency to the catalyst layers and the current collectors to supply gases to the catalyst layers, so as to prevent such flooding. Particularly in the case of a polymer electrolyte fuel cell whereby a high output density at a low temperature is desired, it is important to secure sufficient gas supply to the catalyst layers by imparting water repellency to the current collectors.
For example, as a method for imparting water repellency to a current collector made of e.g. carbon paper or carbon cloth, a method of incorporating a fluorine-containing polymer to a current collector, has heretofore been known. The fluorine-containing polymer may, for example, be polytetrafluoroethylene (hereinafter referred to as PTFE), a tetrafluoroethylene/hexafluoropropylene copolymer or a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer. Each of them is a resin not soluble in a solvent. In this specification, an A/B copolymer represents a copolymer comprising polymer units based on A and polymer units based on B.
In order to incorporate such a fluorine-containing polymer as a water repellent material to a current collector, a method may, for example, be employed in which a sheet constituting a current collector is impregnated in a dispersion of a powder of a fluorine-containing polymer, followed by baking at a temperature of about 300° C. Usually, a surfactant is used as a dispersing agent for such a dispersion of a fluorine-containing polymer, and the surfactant is removed by baking. The surfactant is a hydrophilic substance, and therefore, if it is not sufficiently removed, no adequate water repellent effect by the fluorine-containing polymer can be obtained.
In the above method, high temperature baking is required, whereby there will be many restrictions in the design of the electrodes. For example, a catalyst layer usually contains an ion exchange resin for coating the catalyst, and the heat resistant temperature of such an ion exchange resin is about 200° C. Therefore, the above-mentioned baking treatment can not be applied in a state where a catalyst layer is laminated on the current collector, and the current collector will have to be baked alone.
Further, the particle size of the above-mentioned solvent-insoluble fluorine-containing polymer is at least 0.1 μm as a primary particle size, and when it is used as a powder, it is usually granulated, whereby the average secondary particle size is usually at a level of from a few μm to 500 μm. Accordingly, when such a solvent-insoluble fluorine-containing polymer is incorporated to a current collector, if the amount is small, it can not be continuously present, and the current collector will have water repellency only locally.
Therefore, portions of the current collector where no fluorine-containing polymer is present, will gradually be wetted as the fuel cell is used, and wetted regions will spread therefrom, thus leading to a substantial decrease in the water repellency of the entire current collector. Consequently, pores of the current collector will be clogged by water, and there will be a problem that the supply of a gas to the catalyst layer is hindered, and the concentration overpotential increases to substantially lower the output voltage. Further, the above-mentioned fluorine-containing polymer is substantially spherical in its shape, and even when subjected to baking treatment, the bonding strength to the sheet which constitutes the current collector, is weak, whereby there will be also a problem that the fluorine-containing polymer is likely to partially fall off when used for a long period of time.
Therefore, in order to impart adequate water repellency continuously over the entire current collector, a large amount of the fluorine-containing polymer will be required. However, the above-mentioned solvent-insoluble fluorine-containing polymer is electrically insulating, and if it is incorporated in a large amount in the current collector, the resistance of the current collector increases. Further, there is a problem that the fluorine-containing polymer particles themselves are likely to clog the pores of the current collector.
Therefore, it is an object of the present invention to provide a current collector for a polymer electrolyte fuel cell, which has high water repellency as compared with the prior art and which is capable of maintaining adequate water repellency for a long period of time, thereby to provide a polymer electrolyte fuel cell which has a high output density and which provides a stabilized performance for a long period of time.